1. Field of the Invention
The invention relates to high pressure gas discharge lamps, and more specifically, to a fuse for such lamps.
2. Description of the Prior Art
High intensity discharge (HID) lamps include a discharge vessel typically arranged within an outer envelope. The discharge vessel includes a discharge sustaining filling, a pair of discharge electrodes between which a gas discharge takes place during lamp operation, and conductive lead-throughs extending from each electrode through a wall of the discharge vessel to the exterior in a gas-tight manner. For high pressure sodium (HPS) lamps the discharge vessel is typically of a ceramic material such as a polycrystalline alumina whereas for mercury vapor and metal halide lamps the discharge vessel is typically of fused silica (quartz glass).
All HID lamps are operated on a current limiting ballast because of the negative resistance characteristics of the gas discharge in the discharge vessel. In certain combinations of HID lamps and ballasts, a condition may occur in which the operating voltage of the discharge vessel, as measured across its lead-throughs, is reduced and the current through the ballast and lamp is increased. Such a condition may occur, for example, in unsaturated HPS lamps due to sodium loss or in any HPS lamp which develops an arc tube leak.
A well known problem in HPS lamps is the progressive loss of sodium over rated lamp life (approximately 24,000 hours) due to reaction and/or absorption of sodium with the ceramic discharge vessel, the sealing frit sealing the end of discharge vessel, and the emitter material on the electrode.
As compared to saturated HPS lamps, unsaturated lamps have a very small quantity of sodium and mercury selected such that the sodium and mercury are completely vaporized during lamp operation. Any loss of sodium reduces the lamp voltage. Despite great emphasis to eliminate this so-called "sodium clean-up", unsaturated HPS lamps are still subject to the progressive loss of sodium, which causes a gradual decrease in the operating voltage ("voltage slump") of the discharge vessel over rated lamp life.
For a typical unsaturated HPS lamp having an initial operating voltage of about 130 volts, the voltage slump over life is on the order of 25-30 volts. When used with most ballasts, the electric current through the discharge vessel increases as its operating voltage decreases. When the operating voltage slumps enough, there is danger of damage to the ballast, for example over a period of several hundreds of hours, because of the increased current ("over-current") through the lamp and ballast.
It would be desirable to provide a fuse within the lamp outer envelope to disconnect the discharge vessel from the ballast at some predetermined over-current. However, no commercially available HID lamps are provided with such protection. Ballast damage can occur when the over-current has increased to only between about 15 and 30 percent above the nominal lamp current. Typical fuses, such as those which employ a metallic foil that melts apart under excess current, cannot function to reliably disconnect the arc tube with such an over-current. Additionally, HID lamps typically have a current at lamp ignition which is 2 to 3 times the rated lamp current. The current decreases to the normal operating value only after a warm-up period of several minutes. This condition exists because of the time taken for the arc tube voltage to build up. Any fuse in series with the arc tube would need to discriminate between the high current condition during ignition and warm-up (hereinafter referred to as "start-up") and the prolonged high current during lamp operation due to the lamp voltage slump.
The high ambient temperature of about 250.degree. C. due to heat from the discharge vessel and the possibility of a vacuum environment within the outer bulb are other factors to consider for a fuse that is to be placed within the lamp. Commercially available fuses are generally rated for ambient temperature, their fusing capabilities being substantially reduced for higher temperatures. Commercially available fuses for operation at 250.degree. C. do not appear to be available. Furthermore, the typical design requirement for a fuse that it has near zero resistance during normal operation makes it very difficult to design a fuse for this lamp application.
Mounting of the fuse element is also an important consideration, both from the manufacturing and cost point of view, and also because of the effect other lamp components may have on the operation of the fuse. In addition to the discharge vessel and a support frame which supports the discharge vessel, many HID lamps already contain multiple elements within the outer envelope such as, for example, the components of a starting circuit.
In metal halide lamps and mercury vapor lamps, the starting circuit typically includes an auxiliary electrode in the discharge vessel adjacent one of the discharge electrodes, which auxiliary electrode is connected with the opposite discharge electrode through a current limiting resistor. Often a bimetal switch is in series with the current limiting resistor to remove the resistor and the auxiliary electrode after starting and stabilization of the discharge arc. A common starting circuit for HPS lamps includes a glow starter switch in series with a current limiting resistor and a bimetal, all of which are in parallel circuit with the arc tube. Resistors used in this type of HPS starting circuit typically have a resistance of over a hundred ohms and dissipate high power, on the order of several hundred watts. They are electrically disconnected from the arc tube circuit by the glow switch shortly after ignition of the discharge arc, typically within approximately 20 seconds after initial application of an electric potential to the lamp. Thus, heat from the starting resistors in addition to that from the discharge vessel must also be considered when selecting and mounting a fuse in these lamps.
Thick film resistors are suitable for starting circuits because they reliably dissipate the required several hundred watts for the period just prior to lamp starting (.congruent.20 sec) while having a long life. These are known, for example, from U.S. application Ser. No. 07/378,879 filed Jul. 12, 1989 shows a thick film resistor in a starting circuit for high pressure sodium discharge lamps. J.P. Kokai 56-73856 discloses a thick film resistor as a starting resistor for metal halide lamps and high pressure sodium discharge lamps.
Additional resistors may be electrically connected to the discharge vessel during lamp operation to improve lamp performance. For example, U.S. Pat. No. 4,258,288 (Michael et al) discloses a metal halide lamp for connection to a constant wattage (CWA) ballast of the type having a transformer with a secondary winding in series with a capacitor. The lamp has an internal voltage-doubler starting circuit with two resistors in series with a bimetal switch. The bimetal switch disconnects the starting circuit and auxiliary electrode after starting of the lamp. The lamp also has a third power resistor in series with the discharge vessel which reduces the phase shift between the lamp voltage and the ballast open circuit voltage during lamp warm-up. The resistor increases the maximum sustaining voltage to the lamp when the lamp current is zero, thereby preventing flicker and extinguishment of the arc.
Japanese Kokai 1-211896 shows an unsaturated HPS lamp suitable for operation on a CWA ballast. The lamp has a resistor in series with the discharge vessel to reduce the reignition voltage of the discharge voltage to prevent flicker of the arc, which otherwise occurs under certain operating conditions of the ballast and lamp. Because the resistor is in series with the discharge vessel, it operates continuously during lamp operation after ignition of the discharge arc and dissipates considerable power, approximately 15 watts for a 150 watt lamp.